1. Field of the Invention
The present invention relates to a zoom lens system and an optical apparatus including the same. The present invention is suited for, for example, a single-lens reflex camera, a digital still camera, a camcorder, and a projector.
2. Description of the Related Art
The density of pixels of an image pickup device for use in an image pickup apparatus (optical apparatus), such as a digital camera or a camcorder, is increased more and more. With this, it is desired that an image taking lens (image pickup optical system) for use in such an image pickup device with an increased density of pixels have high resolution.
In an image pickup apparatus (e.g., a digital camera or a camcorder), various kinds of optical members, such as a low-pass filter and an infrared cutoff filter, are disposed between the backmost end of a lens system and an image pickup device. This requires that an image pickup optical system have a relatively long back focus.
Additionally, because variations in characteristics of an incident angle in an image pickup device used in this type of image pickup apparatus are large, the image pickup optical system needs to have excellent telecentric characteristics at the image side.
Examples of an image pickup optical system that meets these conditions include various zoom lens systems that contain a lens unit having a negative refractive power disposed at an object side and a lens unit having a positive refractive power disposed at an image side, so-called retrofocus zoom lens systems.
One example of this type of zoom lens system is a zoom lens system that includes three lens units, consisting of a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power arranged in this order from the object side to the image side, that has a longer back focus and good image-side telecentric characteristics.
One such example of this type of zoom lens system, which includes three lens units, is a zoom lens system that is compact as the entire system while at the same time achieving high image forming performance by use of an aspherical surface and a cemented lens appropriately arranged, as described in U.S. Pat. No. 6,809,879.
Another example is a projection zoom lens system including three lens units that favorably corrects a lateral chromatic aberration while maintaining good telecentric characteristics by use of an aspherical surface and anomalous dispersion glass appropriately arranged, as described in U.S. Pat. No. 6,014,267.
Still another example is a zoom lens system that favorably corrects residual chromatic aberration while at the same time achieving high magnification and being compact as the entire system by use of an aspherical surface and a diffractive optical surface appropriately arranged in an optical path, as described in Japanese Patent Laid-Open No. 2004-117828.
One example of a zoom lens system that has a long back focus and good image-side telecentric characteristics is a zoom lens system including four lens units, consisting of a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power arranged in this order from the object side to the image side, as described in U.S. Pat. No. 6,850,373 and No. 7,079,328.
One example of a zoom lens system that has a long back focus is a zoom lens system including six lens units, consisting of a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, a lens unit having a negative refractive power, and a lens unit having a positive refractive power arranged in this order from the object side to the image side, as described in U.S. Pat. No. 7,184,221.
In typical zoom lens systems, the occurrence of chromatic aberration and variations in the chromatic aberration caused by zooming increase with an increase in zoom ratio. In particular, large secondary spectrums of axial chromatic aberration and lateral chromatic aberration occur in a telephoto state. Satisfactorily correcting this is difficult.
In contrast, when a lens composed of a material that exhibits anomalous dispersion (hereinafter referred to as an anomalous dispersion material) is used in an optical system, the occurrence of chromatic aberration in a telephoto state can be reduced by its anomalous dispersion effects.
However, simply arranging a lens composed of an anomalous dispersion material in an optical path is not sufficient for correction of chromatic aberration over the entire zoom range and improvement in image quality while increasing the zoom ratio.
To satisfactorily correct chromatic aberration over the entire zoom range and obtain high optical performance while increasing the zoom ratio, it is important to arrange a lens composed of an anomalous partial dispersion material in an optical system and to appropriately set the refractive power and other factors.
In particular, in a retrofocus zoom lens system, which includes a lens unit having a negative refractive power arranged in the forefront, the lens units are arranged asymmetrically with respect to an aperture stop. Therefore, it is important how a lens composed of an anomalous partial dispersion material is used.
The zoom lens system disclosed in U.S. Pat. No. 6,809,879 mentioned above achieves high performance with an entire system that is compact, but unfortunately it has a tendency to exhibit a large amount of axial chromatic aberration in the telephoto end state. The zoom lens system disclosed in U.S. Pat. No. 6,014,267 mentioned above can favorably correct lateral chromatic aberration by using anomalous dispersion glass, but unfortunately it has a tendency to have residual axial chromatic aberration, a so-called secondary spectrum.
In addition, because the refractive index of anomalous dispersion glass is low, from the viewpoint of aberration correction, limitations are imposed on a used position thereof.
The zoom lens system disclosed in Japanese Patent Laid-Open No. 2004-117828 corrects axial chromatic aberration occurring in the telephoto end state by using a diffractive optical element. However, producing a diffractive optical element is more difficult than a typical refractive optical element, thus limiting the use of the diffractive optical element.